Self-biasing grid control corona systems

ABSTRACT

Contrast control in two-electrode electrographic apparatus is achieved by sensing changes in the charging current and automatically correcting the bias voltage or terminating the charge deposition on the receiver sheet. For bias voltage correction, a capacitor is connected between the photoconductive grid and the xerographic backing plate. For charge deposition termination, a signal indicative of charge buildup is compared with an external contrast control signal and the electrical circuit to the backing plate is disconnected when the charge buildup signal equals the external contrast signal.

' Frank 1451 Jan. 25, 1972 5 SELF BIASING GRID CONTROL 3,062,956 11/1962Codichini ..250/49.5 3,335,274 fill 967 Codichini et al ..250/49.5

United States Patent CORONA SYSTEMS Primary ExaminerWilliam F. LindquistAttorney-W. H. J. Kline, P. R. Holmes and L. F. Seebach [57] ABSTRACTContrast control in two-electrode electrographic apparatus is achievedby sensing changes in the charging current and automatically correctingthe bias voltage or terminating the charge deposition on thereceiversheet. For bias voltage correction, a capacitor is connectedbetween the photoconductive grid and the xerographic backing plate. Forcharge deposition termination, a signal indicative of charge buildup iscompared with an external contrast control signal and the electricalcircuit to the backing plate is disconnected when the charge buildupsignal equals the external contrast signal.

5 Claims, 3 Drawing Figures PATENIED m2 5:912

LEE E FRANK INVENTOR. 4M

' AGENT BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to xerography and more particularly to an improvedelectric circuit to control the contrast of xerographic copies.

2. Description of the Prior Art In the art of xerography, a uniformnegative charge is laid down on. a photoconductive grid, by means of acoronagenerating device. The grid can then be exposed to a radiationpattern derived from the subject matter to be reproduced by anyconventional projection technique. Such an exposure will decrease thenegative charge on the photoconductive grid in those areas or regionswhich correspond generally to the bright regions on the original andleave the negative charge substantially unaffected in those regionswhich are dark or substantially opaque. A latent electrostatic chargeimage is then produced upon a receiving sheet by directing an ion streamthrough the photoconductive grid. The ion stream will pass more readilythrough those regions which have been decreased in negative charge byexposure tothe radiation and will be blocked by the charged regions. Thelatent image on the receiving sheet can then be developed by anysuitable xerographic means.

Multiple copies are made by replacing the receiving sheet andredirecting the ion stream through the charged photoconductive grid,without any necessity for reexposure of the subject material beingcopied. However, the charge pattern on the photoconductive grid willdecay-with time after the exposure. In the prior art, the amount of.decay has been determined by merely guessing or averaged over a periodof time. In addition, some compensation must be made for the darkcurrent of the photoconductor. It is therefore desirable to providemeans which will automatically compensate for varia tions in thephotoconductor characteristics by controlling the current passing to thereceiving sheet, or the time of charge buildup on the receiving sheet.

SUMMARY OF THE INVENTION The control system of this invention comprisesa capacitor thatis connected between the photoconductive grid and theconductive backing plate of the electrographic system. The capacitor ischarged to provide a voltage differential between the photoconductivegrid and the receiving sheet. A current source connected in series withthe backing plate sets the total current flowing out of the receiversheet. If excess negative charges flow into the receiver sheet, thevoltage across the current source becomes more negative, making thephotoconductive grid more negative, and thereby reducing the currentflow into the receiver sheet. A constant contrast is thereby achievedwith a constant charge transfer time.

ilo control charge transfer time, a comparator circuit compares a signalderived from the voltage across the current source and a signal set byan external contrast control, the circuit to the backing plate beingopened when the desired contrast has been achieved. The invention, itsobjects and advantages, will be apparent to those skilled in the art bythe de led description of the preferred embodiments presented below.

BRIEF DESCRIPTION OF DRAWINGS electrode electrographic system in whichthe automatic bias control device according to this invention isincroporated;

FIG. 2 is a schematic electrical wiring diagram of one embodiment of thebias control device disclosed in FIG. I; and

FIG. 3 is a schematic electrical wiring diagram of a transfer timecontrol device for use in the electrographic system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1,,thexerographic apparatus comprises support means, such as a backing plate 1for carrying a receiver sheet 2. A constant current source 3, such as anilluminated vacuum photodiode, solid-state constant current diodes, apentode vacuum diode, or a high-voltage transistor, is connected inseries with backing plate 1. A conventional corona-generating means,such as corona source 4 is utilized to produce an ion stream by beingheld at a high DC potential by means of potential sources A and B whenswitch S is closed. A grid means is interposed between corona source 4and backing plate 1 said grid means comprising a first grid 5 which isheld at a DC potential, which is less than that of corona source 4, bymeans of a potential source B. A second grid 6 is completely coated witha photoconductive material and interposed between grid 5 and backingplate 1. Capacitor 7 is electrically connected between grid 6 andbacking plate 1. The grids 5 and 6 can be formed of parallel wires asshown in FIG. I or of a wire mesh.

Initially, corona source 4 is activated by closing switch S with noreceiving sheet 2 being present on plate 1. A uniform negative charge isapplied to photoconductive grid 6, the magnitude of the charge beingcontrolled by the potential of grid 5 and the duration of exposure time.Best results have been obtained when the level of charge onphotoconductive grid 6 produces a voltage differential of about200 voltsbetween grid 6 and backing plate 1. Capacitor 7 serves to maintain thisvoltage differential. Corona source 4 is deactivated by opening switch Sand photoconduc'tive grid 6 is then exposed to a radiation patternderived from the subject material to be copied. The negative chargeonphotoconductor grid 6 is decreased in those areas where radiationstrikes grid 6. A

. charge pattern is formed on grid 6 which is a reproduction of thesubject material, the areas corresponding to dark areas on the subjectmaterial having a higher negative charge than the areas corresponding tobright areas. This exposure to a radia tion pattern can be accomplishedin any manner; for example, by projecting on the grid 6, a light patterngenerated by a film transparency (negative or positive) in aconventional manner,

' After the exposure to the radiation pattern as derived from thesubject material to be copied, a receiving sheet 2 is placed uponbacking plate 1. Corona source 4 is reactivated by closing switch S,thereby directing a stream of. ions through grid 5 and photoconductivegrid 6 to receiving sheet 2. The ion stream is modulated by thedouble-grid structure so that the flow of ions to the receiving sheet isin accordance with the radiation pattern. A latent electrostatic imageis thereby produced on receiving sheet 2, which is a reproduction of thesubject material. Multiple copies of the subject material can be madewithout any additional exposure by placing a new receiver sheet 2 onbacking plate I and modulating the flow of another stream of ionsthrough the double-grid structure. A more complete and detaileddescription of such a double-grid electrographic system is disclosed inmy copending application, Ser. No.492,988, filed Sept. 27, 1965.

After receiving sheet 2 has been processed by any conventionalxerographic technique, adjustment can be made to current source 3 tocorrect for the desired contrast in the final copy. Current source 3 iselectrically in series with backing plate 1 and thus sets the totalcurrent flowing out of the receiver sheet 2 and capacitor 7, that is,the ion current and the capacitive current. Because essentially nocurrent flows to photoconductive grid 6, capacitor 7 does not dischargeand, thus, current source 3 sets the current from receiver sheet 2.Should the copy prove too light, current source 3 can be adjusted toallow a larger charge to be deposited upon receiver sheet 2 during agiven charge transfer time and thus darken the resultant copy.Alternatively, if the contrast of the copy is too great, current source3 can be adjusted to produce a smaller total charge on receiver sheet 2and, thus lighten the resultant copy.

When multiple copies of the subject material are made, changes in thecharge on photoconductive grid 6, with time,

will affect the final copy. As the charge on grid 6 dissipates, a higherpercentage of the ion stream from corona source 4 will penetrate throughthe double-grid structure to receiving sheet 2. Normally, this wouldresult in darker copies because of the constant transfer time. However,in the practice of the invention, this will produce a larger currentflowing out of backing plate 1 and will make the potential acrosscurrent source 3 more negative. The potential across current source 3 iscoupled to photoconductive grid 6 by capacitor 7, thereby making grid 6more negative, and reducing the ion stream passing to the receiversheet. Thus changes in the charge on photoconductive grid 6, with time,which produce a larger current to the receiver sheet 2, areautomatically compensated for by increasing the negative bias onphotoconductive grid 6, thereby reducing the current passing to receiversheet 2.

While the embodiment described above automatically corrects fordissipation of the charge on grid 6 after exposure, it does not insurecorrect contrast on the first copy to be made. Contrast control for thisand subsequent copies can be achieved by altering transfer control timeas in the embodiment shown in FIG. 3, wherein corresponding numbersindicate the same elements as in FIGS. 1 and 2. A normally open switch15 is connected in series with backing plate 1. A highimpedancevoltage-sensing amplifier circuit 9 is connected between current source3 and the common terminal of switch 15 and capacitor 7. The output ofamplifier circuit 9 is connected to the input of a nonlinear responsenetwork 10. The output of network 10 is connected to the input of astandard integrator circuit 11, which provides, as an output signal, oneinput to comparator circuit 12. The second input or operating signal tocomparitor 12 is derived from a variable external contrast control 13.The output or control signal from comparator circuit 12 is connected toground through a relay 14 which actuates switch 15, thereby completingthe series circuit through current source 3.

In operation, the desired contrast is initially set on external contrastcontrol 13 which can be a variable resistor, such as a 'rheostat.Photoconductive grid 6 is charged to a desired potential by closingswitch S and activating corona source 4.

' After corona source 4 is deactivated by opening switch S, grid 6 isexposed to a radiation pattern corresponding to the subject that is tobe copied. Switch S is then closed and corona source 4 reactivated.Switch is normally open. However, the output or control signal fromcomparitor 12 will initially be nonzero and, thus, will energize relay14 and hold switch 15 in a closed condition, thereby completing thecircuit to plate 1. As current flows to plate 1, amplifier 9 senses theresultant voltage across current source 3. Network 10 is utilized totranslate a voltage sensed across current source 3 into the controlcurrent which compensates for the nonlinearity of the current flowingthrough the grids 5 and 6 to backing plate 1. This control current isthen integrated with respect to time by integrating circuit 11, theresulting output signal being indicative of the charge produced onreceiver sheet 2 since charging began. In comparitor circuit 12, thisoutput signal is compared with an operating signal which is indicativeof the contrast desired in the final print, the operating signal beingderived from the setting of the external contrast control 13. When thetotal charge received by receiving sheet 2 reaches that set on theexternal contrast control 13, the control signal from comparator 12becomes zero, thereby deactivating relay l4 and allowing opening ofswitch 15, which has been held in a closed position during charging ofreceiver sheet 2.

When multiple copies are being made, charge decay in grid ,6 willproduce a large ion flow to receiver sheet. 2. Contrast Econtrol isautomatically achieved by the circuitry of FIG. 3 since a larger ionflow will produce a larger voltage across current source 3. The voltageis sensed and converted to the comparable plate current; the current isintegrated and compared with the externally set signal in comparitor 12.A shorter transfer time will result as the charge buildup signal willreach e uality with the external contrast signal in a shorter time.

he invention has been describe in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention. In particular, it is understood that the circuitryillustrated in FIGS. 2 and 3 is only one example of circuitry whichcould be utilized to achieve copy contrast control by use of signalfeedback from a constant current source in the backing plate circuit. Inaddition, the signal feedback illustrated in the embodiments describedabove could be adapted to control uniform charging of a receiving sheetby maintaining a constant charging current to the plate or attaining afixed total charge on the plate. Such a device would result from removalof the subject matter exposure step as described in connection with thepreferred embodiments and thereby allow charging of a receiving sheet 2to a uniform charge density for later discharge in accordance withwell-known xerographic principals.

Iclaim:

1. Apparatus for charging a receiving sheet comprising an electricallyconductive support means for said receiving sheet; corona-generatingmeans arranged in spaced relation to said support means for directing acharging current towards said support means; grid means interposedbetween said support means and said corona-generating means, said gridmeans including a first grid of electrically conductive material and asecond grid of electrically conductive material coated with aphotoconductive material and arranged in spaced, generally parallelrelation to and between said first grid and said receiving sheet; acapacitor electrically connected between said second grid and saidsupport means, said capacitor being charged to some predeterminedvoltage before charging current is directed towards said support meansfor applying a bias voltage to said second grid; and a constant directcurrent supply electrically connected between said support meansandground whereby any change in said charging current will vary the currentpassing to said current supply, thereby varying the bias voltage appliedto said second grid.

2. Apparatus as in claim 1 including control means serially connectedbetween said direct current supply and ground and including means forgenerating an operating signal indicative of a predetermined chargelevel on said receiving sheet to maintain said capacitor connected tosaid support means so long as said charging current is being directedtoward said support means.

3. Apparatus as in claim 2 wherein said generating means includes switchmeans electrically connected in series with said support means,capacitor and second grid, said switch means being movable to a closedposition in response to said operating signal and being maintained insaid closed position thereby until said receiving sheet is charged tosaid predetermined level.

4. Apparatus as in claim 3 wherein said switch means, when in saidclosed position, electrically interconnects said direct current supplyand said capacitor to said support means for automatically correctingsaid bias voltage in accordance with any change in said chargingcurrent. 1

5. Apparatus as in claim 4 wherein said control means, includesvoltage-sensing means responsive to the voltage derived from said directcurrent supply for producing a first signal indicative of said chargingcurrent; integrating means responsive to said first signal forgenerating an output signal; and means for comparing said output signaland said operating signal and for generating a control signal, when saidoutput and operating signals correspond generally in amplitude, torelease said switch means, thereby indicating that said receiv ing sheethas been charged to said predetermined charge level.

1. Apparatus for charging a receiving sheet comprising an electricallyconductive support means for said receiving sheet; corona-generatingmeans arranged in spaced relation to said support means for directing acharging current towards said support means; grid means interposedbetween said support means and said corona-generating means, said gridmeans including a first grid of electrically conductive material and asecond grid of electrically conductive material coated with aphotoconductive material and arranged in spaced, generally parallelrelation to and between said first grid and said receiving sheet; acapacitor electrically connected between said second grid and saidsupport means, said capacitor being charged to some predeterminedvoltage before charging current is directed towards said support meansfor applying a bias voltage to said second grid; and a constant directcurrent supply electrically connected between said support means andground whereby any change in said charging current will vary the currentpassing to said current supply, thereby varying the bias voltage appliedto said second grid.
 2. Apparatus as in claim 1 including control meansserially connected between said Direct current supply and ground andincluding means for generating an operating signal indicative of apredetermined charge level on said receiving sheet to maintain saidcapacitor connected to said support means so long as said chargingcurrent is being directed toward said support means.
 3. Apparatus as inclaim 2 wherein said generating means includes switch means electricallyconnected in series with said support means, capacitor and second grid,said switch means being movable to a closed position in response to saidoperating signal and being maintained in said closed position therebyuntil said receiving sheet is charged to said predetermined level. 4.Apparatus as in claim 3 wherein said switch means, when in said closedposition, electrically interconnects said direct current supply and saidcapacitor to said support means for automatically correcting said biasvoltage in accordance with any change in said charging current. 5.Apparatus as in claim 4 wherein said control means, includesvoltage-sensing means responsive to the voltage derived from said directcurrent supply for producing a first signal indicative of said chargingcurrent; integrating means responsive to said first signal forgenerating an output signal; and means for comparing said output signaland said operating signal and for generating a control signal, when saidoutput and operating signals correspond generally in amplitude, torelease said switch means, thereby indicating that said receiving sheethas been charged to said predetermined charge level.